Soldering methods such as refocused and pulse dot soldering are well known, and they have been applied for electrically connecting, for example, the leads 211 and 221 of electronic components 210 and 220 to respective electrical pathways of a printed circuit board 200 as shown in FIG. 13. In the reflowing method, creamed solder is applied over the respective pathways, and then the leads of the electronic components are positioned. This printed circuit board mounted with the electronic components is heated in a furnace to fuse the creamed solder and thereby to join the leads to the pathways.
The reflowing method has an advantage of high productivity since a lot of electronic components can be soldered on a printed circuit board in one process. However, it has some disadvantages. As the printed circuit board and the electronic components mounted thereon are all heated up, the circuit board tends to warp from the heating, and the method is applicable only for soldering electronic components which have heat resistance up to the melting point of the solder. In addition, it presents a possibility of soldering defect. If one or more of the leads of the electronic components are deformed such that some leads stay above and off from respective electrically conductive pathways of the circuit board when the components are mounted on the board (a case shown with a lead 211a in FIG. 14), then they are not soldered.
On the other hand, the pulse dot soldering method never experiences this kind of soldering defect since a pulse dot soldering iron is pressed on each lead to heat precisely a part to be soldered without overheating adjacent parts. As shown in FIG. 14, the pulse dot soldering iron 250 extends vertically and thins out downward with a contact portion 251 at its lower end, and it receives a large electrical current at a low voltage (e.g., 1,000 A at 3 V) through a wire 255 from a current control unit 260. This electrical current spot-heats the contact portion 251, which has the smallest sectional area and thus the largest electrical resistance. The contact portion 251 after being heated up is pressed downward against the lead 211 to solder it.
In preparation for this soldering, creamed solder 202 (i.e., preparation solder or pre-fusion solder) is applied to electrically conductive pathways 201 of the circuit board 200, and the leads 211 of the electronic component 210 are coated with solder (or plated with gold). Then, after the leads 211 are positioned on the pathways 201, the contact portion 251 is pressed down on each lead 211 to fuse the solder and thereby to join the lead to the respective pathway. In this case, the circuit board 200 is heated only at the leads, so there is no risk of the circuit board 200 deforming from heat. In addition, the pulse dot soldering has an advantage that such deformed lead 211a as described above can be soldered without defect because the contact portion 251 presses the lead 211a to its original or non-deformed condition.
However, the whole apparatus operating the pulse dot soldering iron 250 is large and expensive because it uses a large electrical current, requiring a thick wire 255 with a large electrical capacity, and a complex and expensive current control unit 260. In addition, the pulse dot soldering presents another problem. Since the contact portion 251, through which a large current flows, is pressed onto each lead 211 of the electronic component 210, there is a risk of damaging the component 210.
Since the pulse dot soldering iron 250 must have a small sectional area for the contact portion 251 and a thick wire for electrical current, the shape of the soldering iron 250 does not allow much modification for complexity. Therefore, only one row of leads can be heated in one soldering process by the pulse soldering iron of a soldering apparatus which has only one electrical current source. The electronic component 210 shown in FIG. 13, for example, has four rows of leads 211 in a square. If such electronic components 210 are to be mounted on the circuit board 200, the soldering process must be repeated four times for each component. This is not productive and cost-effective.
Although deformed leads 211a can be soldered as described previously with reference to FIG. 14, this requires that the contact portion 251 be raised after the solder has resolidified. If the contact portion 251 were raised while the solder is still fused, then a lead 211a which has a deformation might peel off from the pathway 201 because of resiliency, causing a defect in the soldering. Therefore, the contact portion 251 is kept pressed to the leads 211 even after the current heating the contact portion 251 is cut off. The temperature of the contact portion 251 is let to decline so that the solder resolidifies before the pulse dot soldering iron 250 is raised to complete the soldering. As such, this system needs to detect the temperature of the contact portion 251, so a thermocouple 253 is provided near the lower end of the pulse dot soldering iron 250, for example, by welding.
When the current flows, only the contact portion 251, which is located at the lower end of the pulse dot soldering iron 250, is heated. Therefore, after the start of the current, the time required for this lower end to attain a predetermined temperature is relatively short. Likewise, after the cutting of the current, the time required for the lower end to regain the original lower temperature is relatively short (about 5 or 6 seconds). Thus, the soldering process can be carried out in a relatively short period. This is a major advantage of the pulse dot soldering.
If this soldering process only involved heating and melting the solder applied at the leads, then it could be carried out by a heater which generates a constant heat (e.g., heater of Nichrome wire) instead of a pulse dot soldering iron. The system would be more advantageous with a constant-heat type heater than with a pulse dot soldering iron because it could be constructed less expensively and controlled more easily. However, in the system with a constant-heat type heater, the whole heating head of the heater is heated, so it takes a long time for the heating head to cool down. If this heating head were pressed to the leads to fuse the solder, then it would take a relatively long time for the heating head to cool down and for the solder to resolidify after the cutting off of the current. Thus, the system would require a long processing time. This is a major reason why a constant-heat type heater has not been used in this type of soldering apparatus.